The standard formation for submarine chases was for three chasers to run abreast. At intervals they would stop, listen, and the wing boats would report the bearing readings of their detections to the unit leader (the middle boat). An officer on the unit leader chaser would then compile the three bearing readings and estimate the location of the sound source, the submarine's location.
The Position Plotter was a device designed to facilitate this.
These images show examples of the device.
Photos courtesy of the Harvey Hayes Family Archives. Thanks to Roy Manstan for the scans, and for arranging permission to post them in The Subchaser Archives.